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I would check it... These HFM will fool you b/c if the HFM module sees an ignition misfire , it automatically cuts fuel to that cyl. injector for CAT protection, so an ignition failure on these seems like a fuel issue.
But fuel pressure is a next step.
On the top of the fuel rail, you will see a test port cap..That is where you can hook up a gauge. That fitting is the same as an R12 fitting, in case you do not have one.
So, first check is to start the engine and let it idle..now, pull the vac line at the regulator and see if any gas is shooting out that port on the reg. There should be none and if there is, the diaphragm is leaking and you need a new regulator. [ raw, unmetered gas getting sucked intio intake.]. Also check for any evidense of fuel in that vac line..again, you want none.
If all is well, then hook up gauge and do this simple 3 part test.
1-Engine @ idle, Vac line ON-- FP should be 3.2-3.6 Bar
2- " " , Vac line OFF-FP should jump to 3.7-4.2 Bar
3-Engine OFF , wait 30 min. -FP should maintain at least 2.5 Bar
*Bar = 14.5 PSI... [ you already know that, but this winds up in archieves]

See what's up w/FP...
..next suspect would be the 3 Resistor under the coils if we have a missing/misfire condition, or a vac leak could fit a Rough running condition.
If intermittant, that could be a sticking EGR [ common on 104] . I have a simple test for that after we see where you are w/FP.
You may aso want to try the DM led/sw [ the built-in one] while at idle in case there are any impending codes...
Are you using Bosch F8DC4 plugs?

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A Dalton